The ability to detect, label, and track a molecular target is a powerful research tool. To date, antibodies are the typical reagents used for such research. Antibodies have high specificity for their target, and often can be used for in vivo as well as in vitro applications. Producing antibodies, however, is a sensitive, expensive, and time-consuming endeavor. Polyclonal antibodies, although highly specific, are limited in supply. Monoclonal antibodies, while extremely useful for molecular biology techniques, require labor intensive, diligent cell culture work. Furthermore, if a specific antibody will be used for in vivo human studies, the antibody may require modification to avoid triggering an unwanted immune response. Despite these drawbacks, antibodies have remained the state of the art in many biological research fields.
Aptamers, or target specific nucleic acid sequences, offer an alternative to antibodies. The advent of in vitro selection of aptamers was key to demonstrating their utility. Aptamers can be selected to recognize virtually any target, and they can be synthetically engineered, greatly reducing cost and time when compared to producing antibodies. The most common method for selecting aptamers is referred to as Selex (Systematic Evolution of Ligands by Experimental Enrichment). In general, the Selex process selects target specific aptamers from a pool of randomly generated nucleic acid oligonucleotides through several rounds of selection and amplification. Despite the fact that aptamers are less expensive to make, do not require animal facilities to produce, and can be selected with simple molecular biology techniques, they remain in the background of biological research. A need exists for better aptamer selection methods, which will further increase the advantage of aptamers relative to antibodies. Moreover, although the Selex method has advanced the field of aptamer selection with respect to the selection of one aptamer, a need exists for methods involving the simultaneous selection of two or more aptamers that recognize distinct epitopes on a target molecule.